<!DOCTYPE html>
<html>
<head>
<style type="text/css">
#child, #separator {
 height: 20px;
 background-color: green;
}
#parent {
 margin-bottom: 20px;
}
#child {
 margin-bottom: 10px;
}
</style>
</head>
<body>
<div id="parent">
 <div id="child"></div>
</div>
<div id="separator"></div>
</body>
</html>
